Southern Ocean Time Series

Summary

SOTS (red star) is located north of the Antarctic Circumpolar Current (ACC), where Subantarctic Mode Waters (SAMW) and Antarctic Intermediate Waters (AAIW) form during very deep winter mixing and carry heat and carbon dioxide into the ocean interior. These processes occur in the upper portion, or limb, of the Meridional Overturning Circulation (MOC). The lower limb forms Bottom Water close to Antarctica. Together these two limbs dominate global ocean exchange of Deep Water with the atmosphere.

The Southern Ocean Time Series Observatory provides high temporal resolution observations in sub-Antarctic waters. Observations are broad and include measurements of physical, chemical and biogeochemical parameters from multiple deep water moorings in the sub-Antarctic Zone southwest of Tasmania. The emphasis is on seasonal and inter-annual variations of lower atmosphere and upper ocean properties and their influence on exchange with the deep ocean. The continuous time-series information allow the study of ocean physics and chemistry, climate change, carbon cycling and biogeochemical controls on marine productivity. These moorings provide cost-effective observations and overcome the infrequent availability of ships in the region.

The Southern Ocean (south of 30°S) is responsible for ~40% of the total global ocean uptake of human-induced CO2 emissions, and 75% of the additional heat that these emissions have trapped on Earth. The Southern Ocean Time Series site is focused on the sub-Antarctic Zone because waters formed at the surface in this region, the Sub-Antarctic Mode and Antarctic Intermediate waters, slide under warmer subtropical and tropical waters and carry this CO2 and heat into the deep ocean, out of contact with the atmosphere. This process also supplies oxygen for deep ocean ecosystems, and exports nutrients that fuel ~70% of global ocean primary production. The sub-Antarctic Zone and these processes are expected to change with global warming but the potential impacts of these changes are not yet known.

The Southern Ocean Time Series (SOTS) site southwest of Tasmania is comprised of a number of elements including a deep ocean sediment trap mooring (SAZ), a surface biogeochemistry mooring (Pulse) and an air-sea flux mooring (SOFS). Located in the sub-Antarctic Zone near 140oE, 47oS, the site is particularly vulnerable to the extreme weather events that typify the area including very large waves, strong currents and severe storms, presenting significant technical and engineering challenges. The Southern Ocean Time Series is an Australian contribution to the international OceanSITES global network of time series observatories and is one of the few comprehensive Southern Ocean sites globally.

Instrumentation and Data

The Southern Ocean Time Series moorings are the Pulse biogeochemistry mooring, the Sub-Antarctic Zone (SAZ) sediment trap mooring, and the Southern Ocean Flux Station.

The Pulse biogeochemistry mooring is used to measure upper ocean carbon cycle and phytoplankton productivity processes. Measured parameters include temperature, salinity, dissolved oxygen, total dissolved gases, nitrate, chlorophyll and turbidity. This mooring also collects water samples for measurements of dissolved carbon and nutrients, and phytoplankton microscopic identification.

The SOFS meteorological tower mooring has dual sets of radiometers, temperature and humidity sensors, precipitation gauges and sonic anemometers, and a pCO2 sensor provided by NOAA providing the measurements necessary for computing air-sea fluxes of CO2, heat, momentum and mass. Surface photosynthetically active radiation and surface UV are also measured, to help assess light available for phytoplankton production.

All three moorings are anchored to the ocean floor 4.5 kilometres below the surface. The SOFS and Pulse moorings are s-tether designs that are longer than this, and correspondingly their surface floats move in large ‘watch circles’. In contrast, the SAZ mooring is a stiff subsurface mooring with all components more than 700m below the surface. The moorings record hourly sensor observations until they are swapped with a duplicate mooring the following year.

Data collected from the Pulse and SOFS are relayed back by satellite. The sub-surface data is stored and downloaded when the moorings are retrieved (approximately a year later). All data is available via the Australian Ocean Data Network (AODN) Portal.

The animation below (created by the Marine National Facility) shows the process of deploying moorings and describes what the different types of moorings measure.

Application of Data

Among the key tasks for the maiden voyage in March 2015 of the RV Investigator into the Southern Ocean was the redeployment of three high-precision deep-water moorings as part of IMOS. These moorings are capable of measuring a large array of ocean properties including temperature, salinity, currents, waves, and biological activity, as well as atmospheric conditions. To read the blog updates for Eric Schulz's travels on the maiden voyage click here.

Pteropod shell weights in the high-CO2 Southern Ocean: This study present observations of pteropod shell-weight and flux from 1997–2006 in sediment traps deployed at 47◦ S, 142◦ E at 2000 meters below sea surface in the Southern Ocean. A decadal trend of –1.17±0.47 µg yr−1 (P =0.02) in mean shell weight in the pteropod Limacina helicina antarctica forma antarctica suggests a small but detectable reduction in calcification. Gaps in the data make it difficult to state with certainty the significance of the trend. However, this data set represents the first attempt to estimate interannual variations in pteropod calcification and establish a benchmark against which future impacts of ocean acidification may be detected. Reference: D. Roberts, W. R. Howard, A. D. Moy, J. L. Roberts, T. W. Trull, S. G. Bray, and R. R. Hopcroft. 2008. Interannual variability of pteropod shell weights in the high-CO2 Southern Ocean. Biogeosciences Discuss., 5: 4453–4480 To read more click here.

Latitudinal and temporal distributions of diatom: This study examines two sediment trap records of the flux of diatoms and bulk components collected by two bottom-tethered sediment traps deployed at mesopelagic depths (1 km) in the SAZ (two-year record) and in the PFZ (six-year record) along the 140◦E meridian. These traps provide a direct measure of 10 transfer below winter mixed layer depths, i.e. at depths where effective sequestration from the atmosphere occurs, in contrast to study of processes in the surface ocean. Reference: A. S. Rigual-Hernández, T. W. Trull, S. G. Bray, A. Cortina, and L. K. Armand. 2015. Latitudinal and temporal distributions of diatom populations in the pelagic waters of the Subantarctic and Polar Frontal Zones of the Southern Ocean and their role in the biological pump. Biogeosciences Discuss., 12: 8615–8690. To read more click here.